Winter in Ontario comes with its fair share of challenges. For homeowners, ice damming is one of the more frustrating ones. These thick ridges of ice at the eaves trap melting snow, forcing water back up under shingles or roof coverings. This can lead to leaks, rot, and long-term structural damage.

A properly designed and built roof should shed snow efficiently. But when heat loss, insulation gaps or poor drainage come into play, ice dams can form. Avoiding ice damming is simple: prevent heat from escaping from the living space into the attic and warming the roof. When you design and build homes to withstand the harshest winters, as DeCarli Contractors does, prioritizing energy efficiency and durability means, among other things, no ice damming.

Why do ice dams form?
Ice dams are the result of an imbalance in roof temperature. When the upper portion of a roof gets warm enough to melt snow, water runs down toward the eaves where the roof stays cold. The water refreezes at the colder edge, gradually creating a thick layer of ice. As this cycle continues, water pools behind the ice dam, creeping up under shingles or other roofing materials. It can then leak down into walls, soaking insulation and causing rot and mildew.

Here are the factors that contribute to ice dam formation:

• Inadequate attic insulation Heat loss through the attic warms the roof deck unevenly
• Poor ventilation Without proper airflow, warm air gets trapped under the roof while the edges stay cold
• Air leakage from the living space Gaps around light fixtures, attic hatches and plumbing penetrations allow warm air into the attic
• Roof design and orientation Some roof shapes or orientations experience more snow accumulation and heat loss, increasing the risk of ice dams

Preventing ice dams: a well-built approach

The best way to deal with ice damming is to prevent it from happening in the first place. Proper construction, insulation and ventilation make all the difference. But if  you are experiencing ice dams, you can make adjustments to stop it from happening. The key is maintaining a consistent roof temperature, allowing snow to remain solid while temperatures are cold, and melt and drain naturally when it’s above freezing outside. Here’s what’s involved:

1. Air sealing: stopping heat loss at the source
Sealing off warm air leaks is the first step, ahead of insulation. Small gaps and cracks in ceilings, attic access points, and around penetrations (such as electrical and plumbing openings) allow heated air to escape into the attic. Over time, this warm air builds up, raising roof temperature unevenly.

Proper air sealing involves:

• Sealing attic hatches with weatherstripping and insulation
• Using airtight recessed lighting instead of older, non-IC-rated pot lights
• Caulking and foaming gaps around pipes, vents and chimneys
• Ensuring vapour barriers are continuous and well-sealed

2. Upgrading insulation to maintain a cold roof
With air leaks sealed, the next step is insulating to keep heat where it belongs—in the living space. A well-insulated attic prevents heat from reaching the roof deck, maintaining a uniform temperature across the surface.

In Ontario, attic insulation must achieve a rating of R-60 to meet Ontario Building Code mandates. Blown-in loose fill or spray foam insulation provides excellent coverage, especially in tricky areas like eaves and corners where heat loss is more common. For loose fill in the attic, you will require 16 inches of blown-in cellulose insulation or 24 inches of loose fill fiberglass insulation to achieve R-60.

3. Proper ventilation: allowing cold air to flow
Even with airtight sealing and quality insulation, ventilation plays an essential role in maintaining a consistent roof temperature. Above the insulation and below the roof, you want the temperature to be the same as, or close to, the ambient outdoor temperature. To achieve this, you need proper insulation and a properly-vented attic. That way, cold outdoor air can circulate under the roof deck, preventing warm air from accumulating. A limiting factor, experienced in Ontario during the winter of 2024/2025, was that vents were covered in four feet of snow. Keeping roof vents open in heavy snowfall is important winter maintenance. 

The key elements of effective attic ventilation are:

• Ridge vents – Allowing warm air to escape from the highest point of the roof
• Soffit vents – Bringing in fresh, cold air at the eaves to keep the attic cool
• Baffles – Keeping ventilation paths open above the insulation, preventing the soffit vents from becoming blocked

The balance between intake (soffit vents) and exhaust (ridge vents) ensures that warm, moist air doesn’t get trapped in the attic. This will reduce both ice damming and condensation issues.

Addressing ice dams: what to do if you already have them
If ice dams have already formed, it’s important to address them if they are causing leakage into the building in order to prevent further damage.

In the short term, you can carefully remove snow from the roof using a roof rake, starting at the eaves and working upward to prevent additional meltwater from backing up behind the ice dam. You can also create channels in the ice using warm water or specialized de-icing products to allow trapped water to drain. In extreme cases, where leaks are occurring, professional steaming services can safely remove ice dams without damaging the roof.

In the longer term, you will need to ensure that your attic is air-sealed, well-insulated, and properly ventilated. That’s a project for late spring onwards.

Building for long-term durability
At DeCarli Contractors, we design and renovate homes with ice dam prevention in mind. A well-built roof not only looks great, but also performs well through the toughest winter conditions. From airtight sealing and proper insulation to balanced ventilation, every detail matters.

If you’re dealing with recurring ice damming or planning a new build, now is the time to invest in a solution for the long-term. We would be happy to talk about how to make your roof work for your climate. Please get in touch at your convenience.